Herd immunity sounds tempting. But Bay Area experts warn it would be a disaster – San Francisco Chronicle
Herd immunity is a tempting concept as the United States plods through a pandemic response that has flattened the economy but not the curve, leaving schools and businesses in limbo even as the death toll continues to climb.
Why not let the virus burn through the population unfettered, until enough people have been infected that it can no longer spread? Protect the vulnerable as best as possible, some argue, and get this pandemic over with.
But herd immunity is a dangerous goal that would sacrifice tens of thousands of lives nationwide, especially among people of color, public health experts say. Without the aid of a vaccine, herd immunity may not even be possible with this virus.
Almost no infectious disease or public health experts believe that natural herd immunity — established by people being infected with the virus, not by vaccination — is a reasonable target in this pandemic. But the idea has been batted around anyway, by people frustrated with the economic toll of prolonged shutdowns, or those who are convinced that the country is failing so badly with the pandemic that herd immunity may end up the only outcome.
“It takes a long time to get to herd immunity through natural means. It would take many years,” said Lee Riley, an infectious disease expert at UC Berkeley. “And to achieve it there would be negative consequences. Many people are going to have to die for that to happen. It would just not be ethical to let it happen.”
Sweden infamously gave it a shot. Federal officials there said their goal wasn’t herd immunity, but they never issued widespread shutdowns or mask orders, and their stated plan was to let the virus spread among Sweden’s healthy population while protecting the most vulnerable, and preserving the economy.
The result: Sweden ended up with one of the highest COVID-19 death rates in Europe, it still suffered enormous economic setbacks, and it didn’t come close to reaching herd immunity levels of infection. Studies suggest less than 10% of the population has been infected so far.
“There are people who say we should have done this like Sweden, never go to lockdown,” said Monica Gandhi, an infectious disease doctor and researcher with UCSF.
She said she understands their concerns. Millions have lost jobs and face eviction or not being able to feed their families. Parents are worried their children are losing irreplaceable months of education. Countless people are struggling with the mental health consequences of extreme isolation.
“The reason we didn’t do (what Sweden did) was to avoid the spike in deaths,” Gandhi said. “We do anything we can to drive down rates of severe illness and protect our vulnerable.”
Protecting the human herd: Herd immunity occurs when enough of a population is immune to a virus or other contagious pathogen that disease stops passing from person to person. If a virus can’t find enough people to infect, it will eventually fade away.
Most people are familiar with the concept as it relates to vaccine-preventable diseases such as measles. In the United States, enough people have been vaccinated against measles that the country has achieved herd immunity and the virus can’t spread freely. Outbreaks can still occur, but only if the measles virus is brought in from other countries and under-vaccinated populations are exposed to it.
Vaccination speeds up the process of developing herd immunity and makes it more durable. Measles requires more than 90% of the population be immune to get to herd immunity, which would be nearly impossible to get to and maintain without a vaccine.
Natural herd immunity, from people being exposed to a virus and developing an immune response to it, sometimes occurs in small communities, such as close-knit religious groups or neighborhoods, or contained locations like prisons. San Quentin State Prison, where more than two-thirds of the inmate population has tested positive for the coronavirus, may have reached herd immunity, for example. The remaining third of the population that hasn’t tested positive may be protected because their previously infected contacts provide an immune buffer from the disease.
But in a large population — the size of California, for example — it’s very hard to achieve because it would take years for most viruses to run out of people to infect. The 1918 influenza pandemic took three years to dissipate through herd immunity, in the era before vaccines.
And consider this: Seven months into the COVID-19 pandemic, experts believe that in the hardest-hit parts of the U.S., such as New York City, only about 20% of people have been exposed to the virus and are possibly immune. In the Bay Area, it’s probably under 10% — even after the recent surge in cases.
A handful of infectious disease experts believe that the coronavirus could naturally reach a low-level plateau that mimics herd immunity with a relatively small percent of the population immune. A Stanford epidemiologist has developed a model that suggest it could be as low as 30%.
But based on how easily the new coronavirus spreads, most infectious disease experts believe roughly 60% to 80% of the population would need to be immune — either by vaccine or natural exposure — to achieve herd immunity.
“Outside of New York, I don’t know of any area in the United States where people are realistically saying more than 10% or 15% of the population has been infected,” said Art Reingold, a UC Berkeley infectious disease expert. “And certainly it’s much lower than that in many places. You’re talking about quite a large number of additional infections, hospitalizations and deaths to get to herd immunity.”
Immunity may not be an option: With the virus that causes COVID-19, scientists don’t know if herd immunity is possible. It’s still not clear whether people infected with the virus are then protected from reinfection, and if they are, for how long. Even if infection does offer some protection, most infectious disease experts are skeptical that it’s very long-lasting.
Achieving natural herd immunity to the coronavirus is based on a tower of assumptions, many of which don’t appear to hold up under inspection, said Yvonne Maldonado, a Stanford infectious disease expert and medical director for infection control at Lucile Packard Children’s Hospital.
“You’re assuming that if you’re exposed you will develop antibodies to the virus, which doesn’t always happen,” she said. “And you’re assuming those antibodies will last for some time, which apparently doesn’t really hold true. And you’re assuming those antibodies are even protective, which we just don’t know.”
It’s possible that short-term protection is contributing to herd immunity in small populations, such as San Quentin, where the explosive outbreak dropped off suddenly after peaking about a month ago. Infectious disease experts also point to studies of slums in Mumbai, India, that suggest nearly 60% of residents have been infected and new cases have subsequently fallen.
Some public health officials also have speculated that lower levels of immunity in places such as New York City may be providing some “buffering” that is helping slow down the spread of disease. “This transient, temporary herd immunity that interferes with virus transmission at a highly localized level may delay the spread,” said Shannon Bennett, chief of science with the California Academy of Sciences.
Racial inequities in immunity: But relying on even that level of protection to slow down the pandemic is fraught, Bennett and other experts said. One critical downside is that the virus is not spreading evenly among populations. Communities of color — and in California, Latinos in particular — are carrying a disproportionate burden of disease and death.
“You reopen the economy on the backs of Black and brown people. That’s how this plays out,” said George Rutherford, a UCSF infectious disease expert. If California were to lift restrictions and try to develop some level of herd immunity, “ideally you’d do it in a way where the risk was spread evenly. But I’m not sure how exactly you do that.”
Gandhi, the UCSF researcher, found a note of optimism as the country winds its way through the pandemic and closer to at least partial herd immunity: Masks may help get us there without unduly sacrificing people to severe illness and death.
She said studies increasingly show that severity of illness is tied to how much of the virus people are dosed with, and further research suggests that face coverings — worn by the infected person or the uninfected person, and ideally both — can lower that dose.
Therefore, face coverings could allow more people to get sick with mild or even asymptomatic disease on the way toward developing herd immunity.
That’s dependent on several assumptions, she noted — in particular, that people with mild or asymptomatic disease develop any lasting immunity at all. But given the need to reopen the economy and live with this virus for a while longer, until a vaccine comes along, it would be nice to build herd immunity and protect people at the same time, she said.
“Is there a way to burn through this and give immunity without all the illness? That’s the theory,” Gandhi said. “The idea of masking to get through it and having lower rates of severity of disease, and then immunity — that would be very positive.”
Erin Allday is a San Francisco Chronicle staff writer. Email: firstname.lastname@example.org Twitter: @erinallday
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